Flow control for multiple stacks

ABSTRACT

A fuel cell comprises an electrode plate having a flow field formed therein and a proton exchange membrane. A compressible permeable diffusion media is disposed adjacent the electrode plate. The diffusion media is compressed against the electrode plate so that a portion of the media intrudes into the flow field. A fuel cell stack can be made by compressing a plurality these fuel cells together. The fuel cell stack is compressed so that the diffusion media in each fuel cell is compressed against the adjacent electrode plate with a portion of the media intruding into the flow field in the adjacent electrode plate. The compression of the fuel cell stack can be adjusted so that a magnitude of intrusion of the diffusion media into the flow channels is adjusted and a pressure drop of a predetermined magnitude occurs across the fuel cell stack at a desired operational state.

FIELD OF THE INVENTION

[0001] The present invention relates to fuel cells and more particularlyto controlling the flow of reactants through a fuel cell.

BACKGROUND OF THE INVENTION

[0002] Fuel cells have been used as a power source in many applications.For example, fuel cells have been proposed for use in electricalvehicular power plants to replace internal combustion engines. In protonexchange membrane (PEM) type fuel cells, hydrogen is supplied to theanode of the fuel cell and oxygen is supplied as the oxidant to thecathode. PEM fuel cells include a membrane electrode assembly (MEA)comprising a thin, proton transmissive non-electrically conductive,solid polymer electrolyte membrane having the anode catalyst on one faceand the cathode catalyst on the opposite face. The MEA is sandwichedbetween a pair of non-porous, electrically conductive elements or plateswhich (1) serve as current collectors for the anode and cathode, and (2)contain appropriate channels and/or openings formed therein fordistributing the fuel cell's gaseous reactants over the surfaces of therespective anode and cathode catalysts.

[0003] The term “fuel cell” is typically used to refer to either asingle cell or a plurality of cells (stack) depending on the context. Aplurality of individual cells are typically bundled together to form afuel cell stack and are commonly arranged in electrical series. Eachcell within the stack includes the membrane electrode assembly (MEA)described earlier, and each such MEA provides its increment of voltage.A group of adjacent cells within the stack is referred to as a cluster.

[0004] In PEM fuel cells, hydrogen (H₂) is the anode reactant (i.e.,fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen canbe either a pure form (O₂) or air (a mixture of O₂ and N₂). The solidpolymer electrolytes are typically made from ion exchange resins such aperfluoronated sulfonic acid. The anode/cathode typically comprisesfinely divided catalytic particles, which are often supported on carbonparticles, and mixed with a proton conductive resin. The catalyticparticles are typically costly precious metal particles. As such, theseMEAs are relatively expensive to manufacture and require certainconditions, including proper water management and humidification andcontrol of catalyst fouling constituents such as carbon monoxide (CO),for effective operation.

[0005] The electrically conductive plates sandwiching the MEAs maycontain an array of grooves in the faces thereof that define a reactantflow field for distributing the fuel cell's gaseous reactants (i.e.,hydrogen and oxygen in the form of air) over the surfaces of therespective cathode and anode. These reactant flow fields generallyinclude a plurality of lands that define a plurality of flow channelstherebetween through which the gaseous reactants flow from a supplyheader at one end of the flow channels to an exhaust header at theopposite end of the flow channels.

[0006] Interposed between the reactant flow fields and the MEA is adiffusion media serving several functions. One of these functions is thediffusion of reactant gases therethrough for reacting with therespective catalyst layer. Another is to diffuse reaction products, suchas water, across the fuel cell. In order to properly perform thesefunctions, the diffusion media must be sufficiently porous whilemaintaining sufficient strength. Strength is required to prevent thediffusion media from tearing when assembled within the fuel cell stack.

[0007] The flow fields are carefully sized so that at a certain flowrate of a reactant, a specified pressure drop between the flow fieldinlet and the flow field outlet is obtained. At higher flow rates, ahigher pressure drop is obtained while at lower flow rates, a lowerpressure drop is obtained. However, the pressure drop experiencedbetween the flow field inlet and the flow field outlet may vary from thedesigned pressure drop. Such variations can be caused by variations inthe manufacturing of the fuel cell stacks and/or in the tolerances ofthe components used in the fuel cell stack. Such variations from thedesigned pressure drop can be detrimental to the operation and/orperformance. Therefore, it is desirable to provide a fuel cell and/orfuel cell stack having an improved flow field design.

SUMMARY OF THE INVENTION

[0008] The present invention provides a fuel cell that has a pressuredrop that can be varied. A compressible diffusion media forimplementation with a fuel cell is utilized. The compressible nature ofthe diffusion media enables the pressure drop across the fuel cell to beadjusted so that a desired operation of the fuel cell can be achieved.

[0009] A fuel cell according to the present invention has an electrodeplate with a flow field formed therein and a proton exchange membrane. Acompressible, fluid-permeable diffusion media is disposed adjacent tothe electrode plate. The media is compressed against the electrode plateso that a portion of the media intrudes into the flow field.

[0010] The present invention discloses a method of making an individualfuel cell. The method includes the steps of: (a) positioning acompressible, fluid-permeable diffusion media in between a protonexchange membrane and an electrode plate having a flow field formedtherein; and (b) compressing the diffusion media against the electrodeplate so that a portion of the media intrudes into the flow field.

[0011] The present invention also discloses a method of making a fuelcell stack. The method includes the steps of: (a) positioning aplurality of fuel cells adjacent one another; (b) supplying a feedstream to the plurality of fuel cells; (c) monitoring a pressure drop ofthe feed stream across the plurality of fuel cells; and (d) adjusting acompression of the plurality of fuel cells so that the pressure drop isof a magnitude substantially equal to at least one of a predeterminedrange of pressure drops and a predetermined pressure drop.

[0012] By the present invention it is possible to compensate forpossible variations in the designed pressure drop, it is also possibleto control the amount of pressure drop that is experienced so that arelatively customized operation of the fuel cell can be achieved. Forexample, one pressure drop may be implemented to enhance performance ofthe fuel cell while a different pressure drop may be implemented toenhance an efficiency of the fuel cell system. Additionally, when a fuelcell stack is connected in parallel with one or more other fuel cellstacks such that they all receive a feed stream from a common header, itis possible to adjust the pressure drops of one or more of the variousfuel cell stacks so that the feed stream from the header flows evenlythrough each of the fuel cell stacks. That is, if one fuel cell stackhas a lower pressure drop than other fuel cell stacks that are operatedin parallel, a greater portion of the feed stream will flow through thelower pressure drop stack than through the higher pressure drop stacks.Such variation in the portions of the feed stream that flow through thevarious fuel cell stacks may be controlled or mitigated byimplementation of the invention.

[0013] Thus, it is possible to control and/or adjust the amount ofpressure drop that occurs across the flow fields in the fuel cell and/orfuel cell stack so that specific operational performance can beachieved.

[0014] Further areas of applicability of the present invention willbecome apparent from the detailed description provided hereinafter. Itshould be understood that the detailed description and specificexamples, while indicating the preferred embodiment of the invention,are intended for purposes of illustration only and are not intended tolimit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

[0016]FIG. 1 is an exploded perspective view of a monocell fuel cellaccording to the principles of the present invention;

[0017]FIG. 2 is a partial perspective cross-sectional view of a portionof a PEM fuel cell stack containing a plurality of the fuel cells ofFIG. 1, showing layering including diffusion media;

[0018]FIG. 3 is a detailed view of the portion shown in FIG. 2;

[0019]FIG. 4 is a simplified cross-sectional view of a fuel cell stackbeing compressed according to the principles of the present invention;and

[0020]FIG. 5 is a schematic representation of a plurality of fuel cellstacks operating in parallel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0021] The following description of the preferred embodiment is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

[0022] With reference to FIG. 1, a monocell fuel cell 10 is shown havingan MEA 12 and a pair of diffusion media 32 sandwiched between a pair ofelectrically conductive electrode plates 14. It will be appreciated,however, that the present invention, as described hereinbelow, isequally applicable to fuel cell stacks 15 that comprise a plurality ofsingle cells arranged in series and separated from one another bybipolar electrode plates commonly known in the art. Such fuel cellstacks 15 are shown in FIGS. 4 and 5. For brevity, further reference maybe made to either the fuel cell stack 15 or to an individual fuel cell10, however, it should be understood that the discussions anddescriptions associated with fuel cell stack 15 are also applicable toindividual fuel cells 10 and vice versa and are within the scope of thepresent invention.

[0023] The plates 14 may be formed of carbon, graphite, coated plates orcorrosion resistant metals. The MEA 12 and electrode plates 14 areclamped together between end plates 16. The electrode plates 14 eachcontain a plurality of lands 18 defining a plurality of flow channels 20that form a flow field 22 for distributing reactant gases (i.e. H₂ andO₂) to opposing faces of the MEA 12. In the case of a multi-cell fuelcell stack 15, a flow field is formed on either side of the bipolarplate, one for H₂ and one for O₂. Nonconductive gaskets 24 provide sealsand electrical insulation between the several components of the fuelcell 10. Insulated bolts (not shown) extend through holes located at thecorners of the several components for clamping the fuel cell 10together.

[0024] With particular reference to FIGS. 2 and 3, the MEA 12 includes amembrane 26 sandwiched between an anode catalyst layer 28 and a cathodecatalyst layer 30. An anode diffusion media 32 a and a cathode diffusionmedia 32 c are interposed between the MEA 12 and the plate 14. As shown,H₂ flow channels 20, forming the anode side H₂ flow field, lieimmediately adjacent the anode diffusion media 32 a and are in directfluid communication therewith. Similarly, O₂ flow channels 20, formingthe cathode side O₂ flow field, lie immediately adjacent the cathodediffusion media 32 c and are in direct fluid communication therewith.The membrane 26 is preferably a proton exchange membrane (PEM) and thecell having the PEM is referred to as a PEM fuel cell.

[0025] In operation, the H₂-containing reformate stream or pure H₂stream (fuel feed stream) flows into an inlet side of the anode sideflow field and concurrently, the O₂-containing reformate stream (air) orpure O₂ stream (oxidant feed stream) flows into an inlet side of thecathode side flow field. H₂ flows through anode diffusion media 32 a andthe presence of the anode catalyst 28 causes the H₂ to break intohydrogen ions (H⁺), with each giving up an electron. The electronstravel from the anode side to an electric circuit (not shown) forenabling work to be performed (i.e. rotation of an electric motor). Themembrane layer 26 enables the H⁺-ion to flow through while preventingelectron flow therethrough. Thus, the H⁺-ions flow directly through themembrane to the cathode catalyst 28. On the cathode side, the H⁺-ionscombine with O₂ and the electrons returning from the electric circuit,thereby forming water.

[0026] Still referring to FIGS. 2 and 3, flow channels 20 and MEA 12 areshown. Flow channels 20 are sized to have a specific flow area 34through which the feed streams flow. The flow area 34 is sized so thatat a certain flow rate of the feed streams through the flow channels 20,a specific pressure drop occurs across the flow field 22. That is, at acertain flow rate the gaseous reactants flowing through the channels 20will experience a pressure drop between an inlet and an outlet of theflow field 22. The flow rate of the feed streams through the flow field22 may vary depending upon the operation of the fuel cell stack 15, suchas when higher or lower power output is required. At times, it may bedesirable to alter the specific pressure drop that occurs across flowfield 22 for a specific flow rate of a feed stream.

[0027] To change the pressure drop that occurs across flow field 22 fora specific flow rate of a feed stream, diffusion media 32, as shown inFIGS. 2 and 3, is compressible and can be compressed into flow channels20 of flow field 22. Specifically, MEA 12 is compressed between adjacentelectrode plates 14 so that a portion 36 of compressible media 32intrudes into flow channels 20. As shown in FIGS. 4A and 4B, fuel cellstack 15 is compressed by an adjustable compressing member 38 thatapplies a compressive force F which causes the plurality of fuel cells10 to compress together and causes compressible diffusion media 32 tocompress and intrude into flow channels 20 of flow field 22. Preferably,compressible media 32 elastically deforms between about 0-50%. Morespecifically, a cross-sectional area of compressible media 32 preferablyelastically deforms between about 0-50%. The intrusion of portions 36 ofdiffusion media 32 into flow channels 20 reduces flow area 34. Reductionin flow area 34 restricts flow of a feed stream through flow channel 20and flow field 22. The restriction causes an increased pressure drop tooccur for a given flow rate of the feed stream. The amount of intrusionof media 32 into flow channels 20 is dependent upon a variety offactors, such as the specific characteristics of diffusion media 32, thegeometry/dimensions (depth and width) of flow channels 20 and the amountof force F applied. The variable restriction of the flow channels 20allows for control of a feed stream flowing through flow channel 20.

[0028] Diffusion media 32, as was stated above, is used as both an anodediffusion media 32 a and a cathode diffusion media 32 c. Diffusion media32 can be compressible or non-compressible at the typical forces F thatare applied to fuel stack 15. Typically, fuel cell stack 15 iscompressed an amount that causes a pressure in a range between about25-200 psi to be experienced across a total cross-sectional area of fuelcell stack 15. Because of gaps, voids and spaces in the variouscomponents that comprise the fuel cells 10 and the fuel cell stack 15,only about 50% of the total cross-sectional area is typically in contactwith other components. Therefore, a typical fuel cell stack 15 iscompressed an amount that causes a compressive force or pressure in arange between about 50-400 psi to be experienced by fuel cell stack 15.It should be understood, however, that other compressive forces can beapplied and still be within the scope of the present invention. Itshould also be understood that the terms “compressible” and“non-compressible” as used herein are relative terms that are used todescribe the ability of one diffusion media 32 to be compressed andintrude into flow channels 20 at the range of compressive forcesexpected to be encountered in a fuel cell stack 15, relative to anotherdiffusion media 32 not intruding into flow channels 20 any significantamount at the same range of compressive forces expected to beencountered. A significant amount of intrusion into flow channel 20 isthat which allows a flow in the flow channel to be adjusted andcontrolled as described herein. In other words, non-compressibleindicates the media has essentially no discernable or functional effecton the flow through the channel.

[0029] As was stated above, diffusion media 32 can be provided in eithera compressible form or a non-compressible form, depending upon theapplication and design specifications for the fuel cell 10. Preferably,only one of the diffusion media 32 a or 32 c is compressible while theother is non-compressible. By having only one type (anode or cathode) ofdiffusion media 32 compressible, one set of flow channels 20 can besized for a specific pressure drop at a given flow rate while the otherset of channels 20 have a flow area 34 that will vary with thecompression of the fuel cells 10. This in turn allows for the operationof the fuel cell stack 15 to be adjusted to a desired operation, as willbe discussed below. It should be appreciated, however, that bothdiffusion media 32 a and 32 c can be compressible and still be withinthe scope of the present invention. It should also be understood thatnot all of the fuel cells 10 that comprise fuel cell stack 15 need tohave a compressible media 32 to be within the scope of the invention.That is, the number of fuel cells 10 that have a compressible media 32that comprise fuel cell stack 15 can vary depending upon the design ofthe fuel cell stack 15. Therefore, fuel cell stack 15 can include somefuel cells 10 that do not have a compressible media 32 and still bewithin the scope of the present invention.

[0030] The choice of whether to have a compressible anode diffusionmedia 32 a or a compressible cathode diffusion media 32 c will dependupon a desired operation and control of fuel cell stack 15. For example,when the fuel supplied to the fuel cell 10 is an H₂-containing reformatestream from a reforming system, it is preferred to adjust flow area 34in anode flow channels 20 by providing a compressible anode diffusionmedia 32 a. The use of a compressible anode diffusion media 32 a enablesthe amount of reformate fuel flowing through the anode flow channels 20to be accurately controlled. This is preferred because reformate fuel istypically provided by an onboard reforming system that uses energyproduced by the fuel cell system to generate the reformate fuel. Sinceenergy is being expended to produce the reformate fuel, it is preferredto supply only the needed (required) amount of reformate fuel tominimize any waste. The reduction in the amount of reformate fuel in theanode exhaust (waste) allows for more efficient operation of the fuelcell system within which the fuel cells 10 operate. Therefore, when areformate fuel is used, it is preferred that anode diffusion media 32 abe compressible while cathode diffusion media 32 c be non-compressible.

[0031] In contrast, when the fuel feed stream is H₂ from an onboard H₂storage tank, it is preferred to adjust flow area 34 in cathode flowchannels 20 by providing a compressible cathode diffusion media 32 c.This is preferred because little or no energy is consumed by the fuelcell system to provide the H₂ fuel feed stream from the storage tankwhile energy from the fuel cell system is used to provide the oxidantfeed stream in the form of compressor work. By controlling the pressuredrop through the cathode flow channels 20 via compressible cathodediffusion media 32 c, the use of the compressed oxidant feed stream canbe minimized and/or optimized so that energy loss associated with excesscompressor work is minimized. Additionally, by controlling the flowthrough the cathode flow channels 20, it is easier to keep fuel cellstack 15 humidified.

[0032] With respect to the performance requirements of diffusion media32, along with being compressible or non-compressible, diffusion media32 should be sufficiently electrically conductive, thermally conductiveand fluid permeable. The fluid permeability of diffusion media 32 mustbe high for transporting reactant gas and/or H₂O under lands 18 disposedbetween flow channels 20, the electrical conductivity must be high totransport electrons over flow channels 20 from lands 18 to MEA 12 andthe thermal conductivity must be sufficient to transfer heat to theplate which is then dissipated through coolant in contact with theplate.

[0033] Diffusion media 32 enables the diffusion of the reactants (i.e.,H₂ and O₂ ), as well as the reaction products (i.e., H₂O) therethrough.In this manner, the reactants are able to flow from flow channels 20,through diffusion media 32 and into contact with their respectivecatalysts for enabling the required reaction. As described previously,one product of the reaction is H₂O. The redistribution of H₂O acrossfuel cell 10 is of significant importance to the performance of fuelcell 10. Diffusion media 32 enables the flow of H₂O therethrough, frommore hydrated areas to drier areas for homogeneously hydrating fuel cell10. Further, the flow of electrons is also a significant factor in theperformance of fuel cell 10. Inhibited electron flow results in poorperformance and inefficiency.

[0034] Non-compressible diffusion media having the above statedcharacteristics, such as 060 TORAY® carbon paper, are known in the artand will not be described further. A compressible media 32 having thesecharacteristics can be made from a variety of materials. For example, awoven carbon paper, such as V3 elat single side diffuser available fromE-TEK division of De Nora N. A. of Sommerset, N.J., and CF clothavailable from SGL Carbon AG of Wiesbaden, Germany, can be used as acompressible diffusion media. Furthermore, other materials havingsimilar properties to the above mentioned materials can also beemployed.

[0035] Fuel cell stack 15 can be compressed to provide a specificpressure drop for a desired operating state. The specific pressure dropexperienced by a feed stream will vary depending upon a flow rate of thefeed steam through flow channels 20. The variation in the pressure dropwith the flow rate is approximately linear for the pressure drops andflow rates utilized in a typical fuel cell stack 15. Typical pressuredrops are in the range of about 0.1-6.0 psi across the plate. However,other pressure drops can be employed without departing from the scope ofthe present invention.

[0036] The specific pressure drop across fuel cell stack 15 can beadjusted to coincide with a desired operating state of the fuel cellstack 15. When peak power is the most important or critical aspect ofoperation of fuel cell stack 15, compression of fuel cell stack 15 canbe adjusted so that a desired pressure drop across flow fields 22 occurat a specific power output of fuel cell stack 15. To ensure peak powerperformance, the pressure drop is set while fuel cell stack 15 isoperating at a high power level (i.e., 85-100% of peak power), as willbe described in more detail below. When efficiency of fuel cell stack 15is the most important or critical aspect of operation of fuel cell stack15, compression of fuel cell stack 15 can be adjusted so that a desiredpressure drop across flow fields 22 occur at a specific power output offuel cell stack 15. To ensure peak efficiency, the pressure drop is setwhile fuel cell stack 15 is operating at a lower power level (i.e.,10-30% of peak power), as will be described in more detail below.

[0037] The pressure drop that occurs across flow field 22 can be set sothat a minimum velocity of a feed stream flowing through flow fields 22is maintained. Maintaining a minimum velocity is desirable (especiallyat low power operation) to ensure that an adequate shear force ordynamic pressure is generated by the feed stream to transport reactionproducts (H₂O) out of the fuel cells 10 to allow the gaseous reactantsclear access to catalyst layers 28 and 30. The pressure drop can beadjusted so that at a minimum expected flow rate of a feed stream tofuel cell stack 15, a sufficient velocity is maintained through flowchannels 20 such that an adequate shear force or dynamic pressure isgenerated and maintained.

[0038] Compressible diffusion media 32 can be compressed to varyingdegrees as dictated by the application within which the compressiblediffusion media 32 is utilized. It is envisioned that the typicalcompression will be in the range of about 10 to 50%. However, it shouldbe understood that other amounts of compression can be employed withoutdeparting from the scope of the present invention. The actual amount ofcompression will vary depending upon, among other things, the channelgeometry (width and depth of the channels), the desired operation offuel cell stack 15 (desired pressure drop and/or desired flow velocity),and the specific diffusion media used. Electrode plates 14 may employ anelectrically conductive coating that requires compression to effectivelyconduct electricity. That is, the coatings on electrode plates 14exhibit contact resistance and are not sufficiently conductive withoutbeing compressed. The envisioned 10% minimum compression accounts forvariations in the manufacturing and tolerances of the components thatcomprise fuel cell stack 15 and ensures adequate compression and contactbetween compressible diffusion media 32 and adjacent electrode plates 14so that the contact resistance of the electrode plates 14 is less than anominal value. The compression requirements of such coatings can varydepending upon the exact nature of the coating and the design of theplates 14.

[0039] The use of a compressible diffusion media 32 that allows foradjustment to a pressure drop of a feed stream flowing through fuel cellstack 15 enables the pressure drop of a fuel cell stack 15 to beadjusted to match a pressure drop of a different fuel cell stack and/orfor a fuel cell stack 15 to be built to a specific pressure drop orrange of pressure drop. For example, referring now to FIG. 5, aplurality of fuel cell stacks are shown operating in parallel. A firstfuel cell stack 38 is shown operating in parallel with a second fuelcell stack 40 which both operate in parallel with an n^(th) fuel cellstack 42. The fuel cell stacks 38, 40 and 42 operate in parallel suchthat the fuel cell stacks 38, 40 and 42 all share a feed stream 44 froma common feed stream header 46. Each fuel cell stack 38, 40 and 42receives respective portions 48, 50 and 52 of feed stream 44. Thepressure drops across each of the fuel cell stacks 38, 40 and 42 dictatethe flow distribution of the feed stream 44 into portions 48, 50 and 52.That is, the size of portions 48, 50 and 52 received by fuel cell stacks38, 40 and 42, respectively, are determined by the pressure drops of theindividual fuel cell stacks 38, 40 and 42. If the fuel cell stacks 38,40 and 42 have different pressure drops, then each fuel cell stack 38,40 and 42 will receive a different size portion 48, 50 and 52 of feedstream 44. The non-uniform flow distribution to fuel cell stacks 38, 40and 42 may be undesirable.

[0040] To compensate for the variations in the pressure drops of theparallel fuel cell stacks 38, 40 and 42, fuel cell systems have takenvarious approaches. A first approach has been incorporating independentflow metering components that monitor and control the portions 48, 50and 52 received by the fuel cell stacks 38, 40 and 42 so that each fuelcell stack 38, 40 and 42 each receive an adequate portion 48, 50 and 52of feed stream 44. A second approach has been to supply fuel cell stacks38, 40 and 42 with an excessive flow of feed stream 44 so that each fuelcell stack 38, 40 and 42 receives an adequate portion 48, 50 and 52 offeed stream 44. The present invention can compensate for the variationsin the pressure drops of the fuel cell stacks 38, 40 and 42 without thenecessity of having independent flow metering components or supplying anexcessive flow of feed stream 44. The fuel cell stacks 38, 40 and 42when being built can each be compressed so that a pressure dropexperienced across each of the fuel cell stacks 38, 40 and 42 aresubstantially the same or in a common range of pressure drops.Alternatively, one or more fuel cell stacks 15 can be compressed whenbuilt or later have its compression adjusted to have a pressure dropthat substantially matches a pressure drop or range of pressure drop ofan existing fuel cell stack(s) and then used in parallel with theexisting fuel cell stack(s). By balancing the pressure drops across fuelcell stacks 38, 40 and 42, portions 48, 50 and 52 of feed stream 44 aresubstantially the same (all other influencing factors being equal (e.g.,piping restrictions)). Preferably, the pressure drops of each of thefuel cell stacks 38, 40, 42 are adjusted so that they occur atsubstantially the same power output or power output range.

[0041] Referring again to FIG. 4, the assembly and compression of fuelcell stack 15 is shown. A plurality of fuel cells 10 are arrangedadjacent one another into a fuel cell assembly 54. Fuel cell assembly 54is positioned between a pair of terminal plates 56 that are used toconduct electrical current to/from fuel cell assembly 54. A pair of endplates 16 are disposed adjacent terminal plates 56 on either side offuel cell assembly 54. Compressive force F is applied to one or both endplates 16 to compress fuel cell assembly 54. Reactant feed streams areprovided to fuel cell stack 15 and operation of the fuel cell stack 15is commenced. A power output of fuel cell stack 15 along with thepressure drop of one or both of the feed streams across fuel cell stack15 are measured and/or monitored. Additionally, a velocity of the feedstreams flowing through fuel cell stack 15 can also be measured and/ormonitored. The operation of fuel cell stack 15 is adjusted until fuelcell stack 15 is operating at a desired state (e.g., power level).

[0042] Compressive force F is then adjusted in magnitude until fuel cellstack 15 exhibits a desired characteristic. For example, compressiveforce F can be adjusted until the pressure drop across fuel cell stack15 is of a predetermined magnitude, range of magnitudes, or until aminimum flow velocity of one or more of the feed streams through thefuel cell stack 15 is exceeded. The exact operating state of fuel cellstack 15 at the time of adjusting the compressive force F will varydepending upon the desired operation of fuel cell stack 15. For example,as stated above, when peak power output is critical or most important,fuel cell stack 15 may be operated at 85-100% of peak power level whileadjusting the compressive force F. In contrast, when efficiency of fuelcell stack 15 is most important, fuel cell stack 15 is operated at10-30% of peak power level while adjusting compressive force F.

[0043] Once compressive force F (and the associated intrusion of media32 into flow channel 20) has been adjusted to a level that gives adesired operational characteristic of fuel cell stack 15, end plates 16are secured to a pair of side plates 60. Compressive force F is thenremoved. Attachment of end plates 16 to side plates 60 cause end plates16 to remain at a fixed distance apart and maintain the compression offuel cell assembly 54. End plates 16 can be secured to side plate 60 ina variety of ways, as is known in the art. For example, mechanicalfastener 62 can be used to secure end plates 16 to side plates 60. Amore detailed description of the stack compression mechanism illustratedin FIG. 4 is set forth in U.S. application Ser. No. 10/136,781 filed onApr. 30, 2002, which is commonly owned by the assignee of the presentinvention and which disclosure is expressly incorporated by referenceherein. Alternatively, other means for compressing the fuel cell stackwhich provides a generally equalized compression load are known in theart and may be employed with the present invention.

[0044] While flow channels 20 are shown as being generally rectangular,it should be understood that other shapes and configurations that allowcompressible diffusion media 32 to intrude into flow channels 20 anddecrease flow area 34, can be utilized without departing from the scopeof the present invention. Furthermore, while specific pressure drops andpower levels have been used to describe and illustrate the presentinvention, it should be understood that other pressure drops and otheroperational conditions of fuel cell stack 15 and/or fuel cells 10 can beutilized without departing from the scope of the present invention.

[0045] The description of the invention is merely exemplary in natureand, thus, variations that do not depart from the gist of the inventionare intended to be within the scope of the invention. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention.

What is claimed is:
 1. A fuel cell comprising: an electrode plate havinga flow field formed therein; a membrane electrode assembly; and acompressible fluid-permeable diffusion media disposed between saidelectrode plate and said membrane electrode assembly adjacent saidelectrode plate, said diffusion media being compressed against saidelectrode plate so that a portion of said diffusion media intrudes intosaid flow field.
 2. The fuel cell of claim 1, wherein said electrodeplate is a cathode plate.
 3. The fuel cell of claim 1, wherein saidelectrode plate is an anode plate.
 4. The fuel cell of claim 1, whereinsaid diffusion media is compressed against said electrode plate so thata predetermined pressure drop occurs across said flow field.
 5. The fuelcell of claim 5, wherein said predetermined pressure drop occurs at apredetermined power output level.
 6. The fuel cell of claim 1, whereinsaid diffusion media is compressed against said electrode plate so thata velocity of a feed stream flowing through said flow field ismaintained above a predetermined level.
 7. The fuel cell of claim 1,wherein said media is compressed against said electrode plate to causeat least a 10% reduction in the thickness of said diffusion media ascompared to an uncompressed state.
 8. A fuel cell stack comprising: aplurality of fuel cells arranged adjacent one another, at least one fuelcell of said plurality of fuel cells having an electrode plate having aflow field formed therein, a proton exchange membrane electrode assemblyand a compressible fluid-permeable diffusion media disposed between saidmembrane electrode assembly and said electrode plate, said adjacent fuelcells being compressed together so that said diffusion media in said atleast one fuel cell is compressed against said electrode plate with aportion of said diffusion media intruding into said flow field of saidadjacent electrode plate.
 9. The fuel cell stack of claim 8, whereinsaid electrode plates are cathode plates.
 10. The fuel cell stack ofclaim 8, wherein said electrode plates are anode plates.
 11. The fuelcell stack of claim 8, wherein said fuel cells are compressed togetherso that a pressure drop of a predetermined magnitude occurs in a feedstream flowing through said flow fields.
 12. The fuel cell stack ofclaim 11, wherein said predetermined pressure drop is in the range ofabout 0.1 to about 6 psi.
 13. The fuel cell stack of claim 11, whereinsaid predetermined pressure drop occurs at a predetermined power output.14. The fuel cell stack of claim 8, wherein said fuel cells arecompressed together so that a velocity of a feed stream flowing throughsaid flow fields is maintained above a predetermined level.
 15. The fuelcell stack of claim 8, wherein said fuel cells are compressed togetherso that said media in said at least one fuel cell is compressed againstsaid electrode plate to cause at least a 10% reduction in the thicknessof said diffusion media as compared to an uncompressed state.
 16. A fuelcell system comprising: first and second fuel cell stacks arranged inparallel with each fuel cell stack having a plurality of fuel cells andreceiving a portion of a feed stream, at least one fuel cell of saidplurality of fuel cells in said second fuel cell stack comprising: anelectrode plate having a flow field formed therein; a membrane electrodeassembly; and a compressible fluid-permeable diffusion media disposedbetween said membrane electrode assembly and said electrode plate, saiddiffusion media being compressed against said electrode plate so that aportion of said diffusion media intrudes into said flow field.
 17. Thefuel cell system of claim 16, wherein said feed stream is a fuel feedstream and said electrode plate is an anode plate.
 18. The fuel cellsystem of claim 17, wherein said fuel feed stream is a reformate feedstream.
 19. The fuel cell system of said 16, wherein said feed stream isan H₂ feed stream and said electrode plate is a cathode plate.
 20. Thefuel cell system of claim 16, wherein said feed stream is an oxidantfeed stream and said electrode plate is a cathode plate.
 21. The fuelcell system of claim 16, wherein said diffusion media is compressedagainst said electrode plate in said at least one fuel cell of saidsecond fuel cell stack so that a first pressure drop across said secondfuel cell stack is substantially the same as a second pressure dropacross said first fuel cell stack.
 22. The fuel cell system of claim 21,wherein said first pressure drop is associated with a first power outputof said second fuel cell stack and said second pressure drop isassociated with a second power output of said first fuel cell stack, andsaid first and second power outputs are substantially the same.
 23. Thefuel cell system of claim 16, wherein said diffusion media is compressedagainst said electrode plate in said at least one fuel cell of saidsecond fuel cell stack so that a velocity of said portion of said feedstream through said second fuel cell stack is maintained above apredetermined level.
 24. A method of making a fuel cell comprisingpositioning a compressible fluid-permeable diffusion media between amembrane electrode assembly and an electrode plate having a flow fieldformed therein; and compressing said diffusion media against saidelectrode plate so that a portion of said media intrudes into said flowfield.
 25. The method of claim 24, wherein said compressing causes apressure drop of a predetermined magnitude in a stream flowing throughsaid flow field.
 26. The method of claim 25, wherein said pressure dropoccurs at a predetermined power output.
 27. The method of claim 24,wherein said compressing is sufficient to achieve a minimum targetvelocity of a stream flowing through said flow field.
 28. The method ofclaim 24, wherein said compressing of said media against said electrodeplate causes at least a 10% reduction in the thickness of said diffusionmedia as compared to an uncompressed state
 29. A method of making a fuelcell stack comprising the steps of: (a) positioning a plurality of fuelcells adjacent one another, at least one fuel cell of said plurality offuel cells having an electrode plate with a flow field formed therein, amembrane electrode assembly (MEA) and a compressible fluid-permeablediffusion media disposed between said MEA and said electrode plate; and(b) compressing said adjacent fuel cells together so that in said atleast one fuel cell a portion of said diffusion media intrudes into saidflow field of said adjacent electrode plate.
 30. The method of claim 29,wherein step (b) includes compressing said adjacent fuel cells togetherso that a feed stream flowing through the fuel cell stack experiences apressure drop of a predetermined magnitude.
 31. The method of claim 30,wherein said predetermined pressure drop is in the range of about 0.1 toabout 6 psi.
 32. The method of claim 30, wherein step (b) includescompressing said adjacent fuel cells together while the fuel cell stackoperates at a predetermined power output.
 33. The method of claim 29,wherein step (b) includes compressing said adjacent fuel cells togetherso that a velocity of a feed stream flowing through the fuel cell stackis maintained above a predetermined level.
 34. The method of claim 29,wherein step (b) includes compressing said adjacent fuel cells togetherso that a feed stream flowing through the fuel cell stack experiences apressure drop of a magnitude generally equal to a known pressure drop ofa different fuel cell stack.
 35. The method of claim 29, wherein step(b) includes compressing said adjacent fuel cells together so that, at apredetermined power output of the fuel cell stack, a feed stream flowingthrough said fuel cell stack is adjusted toward a value corresponding toa known pressure drop of a different fuel cell stack at a substantiallysimilar power output.
 36. A method of making a fuel cell stackcomprising positioning a plurality of fuel cells adjacent one another;supplying a feed stream to said plurality of fuel cells; monitoring apressure drop of said feed stream across said plurality of fuel cells;and adjusting a compression of said plurality of fuel cells so that saidpressure drop is within a predetermined range of pressure drop values.37. The method of claim 36, wherein at least one fuel cell of saidplurality of fuel cells has an electrode plate having a flow fieldformed therein, a membrane electrode assembly and a compressiblefluid-permeable diffusion media disposed between said membrane electrodeassembly and said electrode plate, and said compressing is sufficient tocause a portion of said diffusion media in said at least one fuel cellto intrude into said flow field in said electrode plate.
 38. The methodsof claim 36, wherein said adjusting is conducted to provide a pressuredrop which corresponds to a predetermined known pressure drop of adifferent fuel cell stack.